Egg moves are based on individual games, not the generation. This is why the same exact egg move and Pokemon can be compatible in HGSS, but not DPP, even though they're both gen 4.
If you really want an Aggron with Head Smash in DPP, you're going to have to breed an Aron with Head Smash in HGSS and then transfer it to DPP via pal pad.
Basically, don't bother trying to breed it with Head Smash in DPP. It won't work.